1. Field
The presently disclosed subject matter relates to devices, systems, and processes useful as a vehicle drivetrain that can includes a primary drivetrain and a secondary drivetrain. The secondary drivetrain can be selectively coupled to the primary drivetrain to provide an on-demand all-wheel-drive drivetrain.
2. Description of the Related Art
Vehicles are typically driven by two wheels, where the remaining wheels (typically two in number, but can be three, four or more) are idle. These two-wheel-drive (2WD) vehicles can be configured either as a front-wheel-drive (FWD) vehicle or a rear-wheel-drive vehicle (RWD). Existing two-wheel-drive (2WD) drivetrains can be supplemented with an on-demand all-wheel-drive (AWD) mode where the idle wheels are driven in addition to the wheels driven in the 2WD mode. That is, in a FWD vehicle, the rear wheels are added to the drive when the drivetrain is in the AWD mode. Likewise, in a RWD vehicle, the front wheels are added to the drive when the drivetrain is in the AWD mode. The AWD mode can be used to increase traction under certain conditions, such as snow, rain, mud, gravel, inclined driving surface and towing. The drivetrain of the vehicle can be switched manually by the driver, or automatically, between the 2WD mode and the AWD mode, as needed.
The fuel economy of a vehicle with an on-demand AWD mode can be less than a similar model of vehicle that is configured with only a 2WD drivetrain. The decrease in fuel economy can be a result of an increase in vehicle weight due to the components of the secondary drivetrain. The decrease in fuel economy can also be a result of an increase in rolling resistance due to rotational inertia and parasitic losses associated with the secondary drivetrain components, even when the drivetrain is in the 2WD mode. In vehicles having known 2WD drivetrains with an on-demand AWD mode, certain secondary drivetrain components (such as a propeller shaft, a plurality of bevel gears, and associated bearings) might be driven either by the engine/transmission or by the idling rear wheels when the vehicle is in the 2WD mode. Thus, the vehicle can experience a decrease in fuel economy when the vehicle is in 2WD mode as compared to a similar model of the vehicle that is configured only with a 2WD drivetrain.
Accordingly, there is a desire to improve the fuel economy, lower component costs, and improve general operational efficiency of a vehicle having a 2WD mode and an AWD mode.